In the presentation of ultrasound images of anatomical features, there is a continuing need to improve the ability to precisely identify tissue/blood boundaries. The availability of images that precisely identify such boundaries enable an examining physician to better discern physical abnormalities. Recently, to improve the contrast between blood-filled regions and tissue regions, ultrasound contrast agents have been used. Such contrast agents are substances which strongly interact with ultrasonic waves, returning echoes which may be clearly distinguished from those returned by both blood and tissue.
Certain contrast agents consist of microbubbles which present a significant acoustic impedance mismatch and a non-linear behavior in certain acoustic fields. Such non-linear behavior is readily detectable through special ultrasonic processing. One type of microbubble contrast agent comprises microbubbles of an inert gas that are coated with a thin biodegradable coating or shell. Such microbubbles are infused into the body and survive passage through the pulmonary system and circulate throughout the vascular system.
The non-linear response of microbubble contrast agents, as contrasted to a relatively linear response from tissue is illustrated in FIG. 1. There, the return from a contrast agent with increasing levels of acoustic transmit power is plotted versus echo magnitude. As can there be seen, the contrast agent echo response exhibits an exponential relationship whereas the response from tissue is approximately linear. Accordingly, at higher acoustic transmit power levels, the contrast agent echo response exhibits a larger difference from the tissue echo response than at lower acoustic transmit power levels.
U.S. Pat. No. 5,577,505 to Brock-Fisher et al., assigned to the same assignee as this Application, describes a method for enhancing the detection of echo returns from microbubbles in circulation relative to tissue. Brock-Fisher et al. achieve increased sensitivity to non-linear responses from the microbubbles by transmitting a first ultrasound signal at a first power level into a region of interest (ROI) to be imaged. The echo responses gathered from that ultrasound signal are stored, and a second ultrasound signal is applied, at a second power level and the ultrasound echo responses stored.
The ultrasound responses are gain compensated and further processed to subtract one from the other so as to remove most of the linear response values from the exponential echo response signal. What remains corresponds to the exponential response portion of the microbubble contrast agent backscatter. The determined difference values are color-coded and added back into the original image in the spatial areas from which the echo signals were generated, enabling better identification of the blood-filled regions carrying the contrast agent.
Notwithstanding the ability to better image the contrast agent in areas of blood circulation, there still is a need to better identify boundaries between tissue and contrast agent-containing regions of an ultrasound image.